Whether the antenna is a capacitive feed type meander line loaded antenna or a standard meander line loaded antenna, these antennas are characterized by their small size and their wide band performance.
Meander line loaded antennas are described in U.S. Pat. No. 5,790,080 issued to John T. Apostolos on Aug. 4, 1998 and incorporated herein by reference. The purpose of the meander line is to increase the effective length of the antenna such that compact antennas may be designed for use, for instance, in cellular phones where real estate for the antenna is limited or in military applications where it is important to be able to provide a compact antenna for surveillance and communications in which the desired frequency range is 30 MHz to 200 MHz.
For cellular applications with the decrease in size of wireless handsets, it is only with difficultly that one can design an antenna which will fit within the margins of the case of the wireless handset and still be useable in dual or trimode phones which span the 830 MHz and the 1.7 and 1.9 Mz bands. Now that GPS receivers are sometimes included in wireless handsets it is important that the antenna also be able to receive the GPS frequency of 1.575 GHz.
As illustrated in U.S. Pat. No. 6,323,814 issued to John T. Apostolos on Nov. 27, 2001 and incorporated herein by reference, an improvement over Apostolos' original patent includes a wideband version in which the meander line loaded antenna has a wide instantaneous bandwidth. In this particular antenna the feed to the antenna is through a meander line coupled between the signal source and a plannar conductor extending orthogonally from the ground plane for the antenna. This configuration offers an instantaneous bandwidth of 7:1 and has been implemented in a so-called quadrature arrangement in which there are two pairs of meander line antennas arranged in opposition. The opposed pairs are orthogonally arranged to enable circular polarization.
As described in this latter patent, the meander line is connected in series between a signal source and a plannar top conductor which is spaced from the ground plane such that the signal from the meander line is directly connected to the top plate. The result for such a feed for the meander line loaded antenna is that the low frequency cut-off of the antenna is determined by the fact that the meander line loaded antenna reactance with a shorted meander line is positive at the lower frequencies, which when added to the meander line and distributed capacity reactance results in a high VSWR at frequencies, in one embodiment, below 860 MHz, thus limiting its usefulness in the cellular band which is centered around 830 MHz.
For military applications, while antennas have been designed to operate between 50 MHz and 200 MHz it is important to lower the low frequency cut off to 30 MHz and still maintain the small size of the antenna. It is noted that in this type of antenna the drive is fed through the meander line and then to the top plate. Moreover, a quadrature arrangement is possible with this meander line design and is desirable when the antenna is mounted to the roof of a truck cab because of the circular polarization provided by the quadrature design.
While capacitive feed meander line antennas have been effective in lowering the low frequency cut off of meander line loaded antennas, there is still a need to operate at even lower frequencies without enlarging the antenna. Whether utilizing a conventional meander line loaded antenna which has an ultra wide bandwidth, or when using a capacitively coupled meander line loaded antenna which in turn has a lowered low frequency cut off, it is desirable to even further lower the low frequency cut off for such antennas, making possible an operating range down to, for instance, 30 MHz in an antennas whose range typically goes from 50 MHz to 200 MHz.
In short, while present techniques permit the lowering of the low frequency cut off of such antenna systems, it is still desirable to have these antennas be able to operate at lower and lower frequencies, yet not increase their size.